Re: [PATCH] scsi: sd: add a capacity_override attribute

[Ewan Milne <emilne@redhat.com>]

On Tue, 2015-03-17 at 14:08 -0400, Alan Stern wrote:
> This patch provides a sysfs interface allowing users to override the
> capacity of a SCSI disk.  This will help in situations where a buggy
> USB-SATA adapter fails to support READ CAPACITY(16) and reports only
> the low 32 bits of the capacity in its READ CAPACITY(10) reply.  For
> an example, see this thread:
> 
> 	http://marc.info/?l=linux-scsi&m=140908235510961&w=2
> 
> The interface is awkward because it requires the user to tell the
> system to re-read the disk's partition table afterward, but at least
> it provides a way to handle deficient hardware.

I think that it is confusing that writing into the capacity_override
sysfs node does not get immediately reflected in the gendisk structure.
Would it hurt to call sd_revalidate_disk() after the value is changed
in capacity_override_store()?

The thing is, if someone overrides the capacity but does not do anything
right away to revalidate the disk, it could change at some arbitrary
time in the future when the revalidation happens for some other reason.

-Ewan

> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> CC: Dale R. Worley <worley@alum.mit.edu>
> 
> ---
> 
> 
> [as1777]
> 
> 
>  Documentation/ABI/testing/sysfs-class-scsi_disk |   19 ++++++++++++
>  drivers/scsi/sd.c                               |   37 ++++++++++++++++++++++++
>  drivers/scsi/sd.h                               |    1 
>  3 files changed, 57 insertions(+)
> 
> Index: usb-4.0/Documentation/ABI/testing/sysfs-class-scsi_disk
> ===================================================================
> --- /dev/null
> +++ usb-4.0/Documentation/ABI/testing/sysfs-class-scsi_disk
> @@ -0,0 +1,19 @@
> +What:		/sys/class/scsi_disk/HOST:CHANNEL:TARGET:LUN/capacity_override
> +Date:		March 2015
> +KernelVersion:	4.1
> +Contact:	Alan Stern <stern@rowland.harvard.edu>
> +Description:
> +		This file provides a way for users to override the
> +		automatically determined disk capacity.  For example, some
> +		buggy USB-SATA adapters report only the low 32 bits of a
> +		drive's block count, resulting in a calculated capacity
> +		value that is the actual capacity modulo 2 TB.
> +
> +		After the correct capacity (in native-size blocks -- often
> +		512 bytes per block but sometimes 4096) is written to this
> +		file, the user must tell the system to re-read the disk's
> +		partition table by running the command:
> +
> +			/usr/sbin/blockdev --rereadpt /dev/sdX
> +
> +		where X is the disk's drive letter.
> Index: usb-4.0/drivers/scsi/sd.h
> ===================================================================
> --- usb-4.0.orig/drivers/scsi/sd.h
> +++ usb-4.0/drivers/scsi/sd.h
> @@ -66,6 +66,7 @@ struct scsi_disk {
>  	struct gendisk	*disk;
>  	atomic_t	openers;
>  	sector_t	capacity;	/* size in 512-byte sectors */
> +	sector_t	capacity_override;	/* in native-size blocks */
>  	u32		max_xfer_blocks;
>  	u32		max_ws_blocks;
>  	u32		max_unmap_blocks;
> Index: usb-4.0/drivers/scsi/sd.c
> ===================================================================
> --- usb-4.0.orig/drivers/scsi/sd.c
> +++ usb-4.0/drivers/scsi/sd.c
> @@ -477,6 +477,35 @@ max_write_same_blocks_store(struct devic
>  }
>  static DEVICE_ATTR_RW(max_write_same_blocks);
>  
> +static ssize_t
> +capacity_override_show(struct device *dev, struct device_attribute *attr,
> +		char *buf)
> +{
> +	struct scsi_disk *sdkp = to_scsi_disk(dev);
> +
> +	return sprintf(buf, "%llu\n",
> +			(unsigned long long) sdkp->capacity_override);
> +}
> +
> +static ssize_t
> +capacity_override_store(struct device *dev, struct device_attribute *attr,
> +		const char *buf, size_t count)
> +{
> +	struct scsi_disk *sdkp = to_scsi_disk(dev);
> +	unsigned long long cap;
> +	int err;
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EACCES;
> +
> +	err = kstrtoull(buf, 10, &cap);
> +	if (err)
> +		return err;
> +	sdkp->capacity_override = cap;
> +	return count;
> +}
> +static DEVICE_ATTR_RW(capacity_override);
> +
>  static struct attribute *sd_disk_attrs[] = {
>  	&dev_attr_cache_type.attr,
>  	&dev_attr_FUA.attr,
> @@ -489,6 +518,7 @@ static struct attribute *sd_disk_attrs[]
>  	&dev_attr_provisioning_mode.attr,
>  	&dev_attr_max_write_same_blocks.attr,
>  	&dev_attr_max_medium_access_timeouts.attr,
> +	&dev_attr_capacity_override.attr,
>  	NULL,
>  };
>  ATTRIBUTE_GROUPS(sd_disk);
> @@ -2152,6 +2182,13 @@ sd_read_capacity(struct scsi_disk *sdkp,
>  	struct scsi_device *sdp = sdkp->device;
>  	sector_t old_capacity = sdkp->capacity;
>  
> +	/* Did the user override the reported capacity? */
> +	if (!sdkp->first_scan && sdkp->capacity_override) {
> +		sector_size = sdkp->device->sector_size;
> +		sdkp->capacity = sdkp->capacity_override;
> +		goto got_data;
> +	}
> +
>  	if (sd_try_rc16_first(sdp)) {
>  		sector_size = read_capacity_16(sdkp, sdp, buffer);
>  		if (sector_size == -EOVERFLOW)
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html